It is well known that an internal combustion cylinder/piston engine requires the proper mixture of air (oxygen) and fuel (gasoline) for optimum performance. Also, the optimum mixture of fuel and air tends to vary as the speed of the engine changes. This is evident when the engine is operating a idle speed and then is rapidly accelerated.
With a high performance engine having a carburetor that controls the mixture of air and fuel to the engine, it is difficult to set the proper amount of air and fuel flow to the engine during idling of the engine. When an engine does not idle fast enough the minimum openings of the butterfly valves of the base plate can be adjusted slightly to remain open and increase the flow of air and fuel to the engine, but this only adjusts the flow of both fuel and air substantially at the same ratio, not adjustment of the air without adjusting the fuel, or vice versa.
It is important for an internal combustion engine to have the proper air and fuel mixture in order for the engine to idle and run properly. When an engine is at idle speed it is likely that a higher air-to-fuel ratio is required to avoid choking of the engine and hesitation (“stumbling”) of the engine when the engine accelerated. If there is too much fuel in the air moving to the combustion chambers, the engine will idle rich. A rich condition produces problems ranging from an exhaust that is foul smelling and hard on the eyes to fouled sparkplugs that affect the performance of the engine. If there is not sufficient fuel present with respect to the air, the engine will idle lean. A lean condition will lead to a hesitation or “stumble” of engine performance when the throttles are opened in an attempt to accelerate the engine. Neither rich nor lean condition is advantageous to the engine.
Fuel is delivered to the engine at idle via the idle circuit of the carburetor. There are multiple adjustments that can be made to allow the carburetor to deliver a given amount of fuel. This fuel is what keeps the engine running until the butterflies open to a sufficient point, allowing the air speed to increase and start the main circuit that feeds the majority of the fuel to the running engine.
The amount of air delivered to the engine at idle generally is controlled by the position of the butterflies. The butterflies sit in the bores of the baseplate of a carburetor assembly. By rotating them from a near flat position (near horizontal) towards a more vertical position, the amount of air delivered to the engine through the carburetor is increased. It is with this butterfly adjustment that a common problem with adjusting carburetors at idle appears.
The carburetors idle circuit breaks down into two basic discharge locations per butterfly bore. Some of the fuel is discharged from the idle discharge hole, which is located below the butterfly, exposed to the depression of pressure caused by the running engine. It is this low pressure that leads to a difference in pressure with the atmosphere that causes air to move through the carburetor and draw fuel to be discharged into the engine. The second discharge location for idle fuel is the vertical transfer slot of the base plate. This vertical transfer slot is located in the path of movement of the upwardly opening butterfly valves, usually extending below the butterfly valves, and are exposed to the low pressure drawn by the engine. The transfer slots supply a small amount of the fuel necessary to keep the engine running at idle, but then act as a reserve fuel supply as the fuel demand of the engine increases in response to the increase of air flow, the increase of air flow responding to the movement of the butterflies to a more vertical position. In the proper closed butterfly position, about 10-15% of the transfer slots are exposed below the butterflies. The opening butterfly sweeps past this vertical slot, producing a low pressure area from the air passing between the edge of the butterfly and the bore of the baseplate which has the transfer slot cut into it. As the butterfly travels, more slot is exposed, thus delivering more and more fuel. The name “transfer slot” is used because this slot delivers the necessary fuel to the engine as the carburetor transfers from the idle circuit to the main circuit.
The problems mentioned above of poor idling and engine stumble usually occur if the butterfly is open too much during idle speed and as a result too much of the transfer slot is exposed to low pressure, drawing more fuel from the transfer slot and there is not enough fuel left in the reserve fuel supply to cover the transition from idle to main circuit operation. One of the greatest causes of this open butterfly condition results when an engine produces very little signal (vacuum) at idle. This condition can be caused by a radical camshaft (camshaft with long duration and/or high lift), large ported cylinder heads, or a large intake manifold. Due to the low vacuum produced by these engines at idle, it may be necessary to open the butterflies from the closed position a greater amount to allow enough air to enter the engine to maintain idle. This generally leads to a rich idle, and a stumble off idle due to the fact that the fuel reserve that feeds the transfer slot is out of fuel.
It is this bad tuning condition that this invention will improve.